As a load-carrying rotor of a helicopter turns in the air, the blades or wings of the rotor provide lift and also develop drag. The drag of the rotor reacts through the rotor shaft and driving mechanism to produce torque tending to turn the fuselage of the helicopter in a direction opposite to that in which the rotor is turning. In a helicopter having a single load-carrying rotor, the reaction torque is customarily counterbalanced by a small rotor having an approximately horizontal axis, for example a tail rotor. In a helicopter having contra-rotating load-carrying rotors, the reaction torque of one rotor is balanced by that of the other so that no tail rotor is necessary. Directional control of such helicopter can be obtained by causing the reaction torque of one rotor to be greater than that of another so that there is a resultant torque differential tending to turn the helicopter fuselage.
It has previously been proposed to provide directional control for a helicopter with contra-rotating load-carrying rotors by varying the pitch of the rotor blades. If the pitch of the blades of one rotor is increased and the pitch of the blades of the other rotor is decreased by the same amount, the drag of the first rotor becomes greater while that of the second rotor is diminished, but the total lift of the two rotors remains essentially the same. This produces a torque differential that reacts through the rotor transmission and appears as a torque tending to turn the fuselage of the aircraft about the approximately vertical axis of the rotor shaft. It is thus possible, by differential control of the rotor blade pitch, to control the directional heading of a helicopter.
The use of differential collective pitch to control the directional heading of a helicopter has been found satisfactory in "power on" flight, i.e. , when the rotors are driven by the engine. However, it has been found experimentally that in "power off" (autorotative) or "partial power" flight, differential collective pitch control is not satisfactory. In moderate to high speed autorotative forward flight, the differential collective pitch control has a tendency to reverse in effectiveness, i.e., application of right rudder by the pilot gives rise to left yaw, and vice versa. In zero speed or low speed autorotative flight with such a control system, the directional control becomes very "soft," and, in the case of coaxial rotors, the helicopter has a strong tendency to turn in the direction of rotation of the lower rotor. These effects present a serious problem in maneuvering the helicopter and give rise to an accident hazard.
Through Ryan et al. U.S. Pat. No. 2,835,331, it is known to obtain directional control of a rotary wing aircraft having contrarotating load-carrying rotors by means of tip brakes on the rotor blades. The term "tip brake" is used to designate an aerodynamic brake device disposed at or near the tip of the rotor blade and operable to increase the drag of the blade, preferably without materially affecting its lift. The tip brakes are operated by a control system actuated by a pilot-controlled steering member which may, for example, be in the form of rudder pedals, a rudder bar, or a steering wheel. The operation of the tip brakes is coordinated by the control system so that the tip brakes of one or another of the rotors are actuated selectively.